Pattern definition, the combination of photolithography and etching, has evolved greatly over the past twenty-five years. The levels of integration we are currently witnessing would be impossible without advanced pattern definition schemes. There have been considerable advances in lithographical exposure methods and in the chemistry of resist materials and etchants, but the development of photoresist remains a wet step even in dry etching processes. However, wet developing is undesirable in advanced integrated circuit fabrication because the organic resist material tends to swell when bathed in the developer. This effect leads to a loss in resolution, a problem which must be avoided for small geometry pattern definition. A further problem with wet developing is the formation of developer residue ("scum"), which will act as a block to subsequent etching. There have been many attempts to attain dry developing materials but the results of these efforts have not yet been totally successful.
There are essentially two different etch methods which are used in conjunction with photolithography; wet etching and dry (plasma) etching. Wet etching is unsuitable for advanced fabrication as the substrates must be immersed in "dirty" chemicals which become more contaminated with each batch, and also must ultimately be disposed of in an ecologically safe manner.
Still further, the extended etch times of wet etching will lead to the failure of resist adhesion at the edges of developed patterns and hence resolution will suffer. Wet chemistry is also not favored for reduced geometry features as surface tension effects will exclude the etchant from small openings. Furthermore, attempts to circumvent this latter problem with surfactants can lead to chemical contamination.
Although plasma etching and its derivative processes such as reactive ion etching, are now universally used for the patterning of many different materials because of their freedom from the problems of wet etching, they are still not ideal as the resists produced thereby are usually etched and frequently become cross-linked. The latter effect tends to make resist removal difficult.
Gas plasmas which etch silicon dioxide also tend to attack the underlying silicon and hence end point detection is critical if shallow underlying junctions are to remain intact. There is also the potential for radiation damage of the sensitive oxide in the energetic plasma. This is an important consideration for gate oxides in metal-oxide-semiconductor (MOS) devices as some types of residual damage will alter device characteristics. From a practical point of view, plasma etching equipment is complex and invariably expensive.
The development of the present invention herein denominated "Carbon Enhanced Vapor Etching" or the "CEVE Process", was stimulated by the work of R. L. Bersin et al and a technique they called "permeation etching". (See: Solid State Technology, April 1977, P. 78) Bersin et al discovered that when silicon dioxide is covered with negative photoresist, it can be rapidly etched with anhydrous hydrogen fluoride, (HF), whereas oxide not covered with resist was relatively impervious to attack. The net effect was image reversal on the oxide as the negative resist was exposed, developed to obtain a negative pattern, and then the oxide was etched underneath the resist, leaving the developed or uncovered regions intact. The effect was attributed to the permeation of the anhydrous HF through the resist, where it reacted with some of the components of the organic material, presumably carbon and hydrogen. The carbon/hydrogen "activated" regions were etched more rapidly than oxide exposed to anhydrous HF alone.
The advantage of the technique was the ability to etch small features with a high degree of etch selectivity over silicon without the need for wet chemistry. However, this technique suffered from several problems, the main one being that a develop step was still required. Also, a plasma etching system was used as the process was performed at low pressure and utilized plasma treatment of the exposed oxide to render it inactive. The process was also carried out at relatively high temperature (above 180.degree. C.) which could have resulted in resist flow or difficulties in resist removal for long etching times.
A more practical approach was attempted by R. X. Pei, (Acta Electonica Sinica, No. 1, 1978, p 102) who noted that hydrofluoric acid vapor, produced by bubbling nitrogen through concentrated hydrofluoric acid, would also permeate through the resist to etch the underlying silicon dioxide. The etching effect was considerably accelerated if the negative resist was exposed but not developed prior to etching. Oxide under exposed regions of resist would etch at a faster rate than oxide under unexposed resist, and hence a positive pattern would be formed on the oxide layer (image reversal). This appeared to be an improved silicon dioxide patterning process, although it suffered from the fact that it was limited to optically exposed photoresist coatings and was not explained relative to other carbon sources or exposure methods.
Accordingly, it is toward the provision of a new and unique carbon enhanced vapor etching process that the present invention is directed.